Consider:

var pq = new PriorityQueue<Number>(new TreeSet<>(Integer::compare))
pq.addAll(Arrays.asList(1.0, 2.0))
==> ClassCastException

The problem is that the copy constructor used above takes a SortedSet<? extends E> which allows the PQ type arg to be a supertype of the SortedSet's type arg. However, the constructor then does

this.comparator = (Comparator<? super E>) c.comparator();

The arg's comparator is Comparator<Integer> but this casts it to Comparator<? super Number> which is clearly unsafe. (Unchecked warnings are suppressed for this entire class.)

A similar problem can occur in the copy constructor that takes a Collection; in that case an instanceof/downcast to SortedSet is performed before the comparator is fetched; the latter is also unsafe.